Published online by Cambridge University Press: 18 December 2008
‘Phalanx’ and ‘guerrilla’ phenotypes have been characterized as distinct, adaptive growth strategies exhibited by marine encrusting taxa, as well as a variety of other colonial taxa, that differ in patterns of colonial growth and areal expansion. Phalanx morphs exhibit compact growth, expanding outward concentrically and generating radially symmetric colony shapes, whereas guerrillas exhibit diffuse growth and typically elongate, asymmetric colony shapes. Several species in the colonial hydroid genus Hydractinia display inter-genotypic morphological variation in early developmental growth, although it is unclear if and how this growth form variation is tied to colony symmetry. Here I show that the phalanx versus guerrilla distinction does not adequately characterize genetic variation in Hydractinia growth form. Genotypes characterized by extreme mat and stoloniferous growth exhibited high levels of symmetry while genotypes generating growth forms intermediate between these two extremes were more asymmetric. Asymmetric growth is tied to reduced field fitness as a result of slower growth, reduced investment in future reproduction, and increased susceptibility to abiotic environmental stress. Asymmetric, guerrilla-like growth may be the morphological symptom of maladaptive growth early in colony development. This notion contrasts greatly with the traditional view of guerrilla growth as an adaptive strategy. Several hypotheses are proposed to address why asymmetric, guerrilla-like growth may be maladaptive in this and similar systems.